1. Field of the Invention
The present invention relates to a two-fluid cleaning jet nozzle and, more specifically, to a two-fluid cleaning jet nozzle for clearing a workpiece such as a semiconductor wafer, or the like, of contaminants adhering to the workpiece. The present invention further relates to a cleaning apparatus and method provided with such a two-fluid cleaning jet nozzle to clean a workpiece, such as a semiconductor wafer of contaminants adhering to the workpiece.
2. Background of the Invention
Generally, various contaminants adhere to the surface of a semiconductor wafer during semiconductor device fabricating processes. For example, dust particles adhere to the surface of an insulating film or a metal film formed on a semiconductor wafer by a CVD process or a sputtering process. When a semiconductor wafer carrying a film or films is subjected to dry etching to pattern the film, resist particles or metal particles remain on the surface of the semiconductor wafer. There have been proposed high-pressure cleaning methods, ice scrubber cleaning methods and liquid jet cleaning methods using a two-fluid cleaning jet nozzle to remove such contaminants from the semiconductor wafer.
FIG. 17 is a view of a conventional high-pressure jet cleaning apparatus. When cleaning a semiconductor wafer 5 by this high-pressure jet cleaning apparatus, the semiconductor wafer 5 is supported on a stage 6 which is rotated by a motor 7. A high-pressure jet nozzle 69 connected to a pressurized liquid supply unit 68 by a pipe is disposed opposite to the semiconductor wafer 5 supported on the stage 6.
When cleaning the surface of the semiconductor wafer 5, a liquid such as pure water is pressurized to 50 to 100 kgf/cm.sup.2 by the pressurized liquid supply unit 68 and is supplied through the pipe to the high-pressure jet nozzle 69. The high-pressure jet nozzle 69 jets the high-pressure liquid continuously through a nozzle hole of about 0.1 mm in diameter against the surface of the semiconductor wafer 5 to remove contaminants adhering to the surface of the semiconductor wafer 5.
The cleaning ability of this high-pressure jet cleaning apparatus, however, is not satisfactory and in some cases the high-pressure jet cleaning apparatus is unable to remove dust particles of 1 .mu.m diameter or smaller. The cleaning ability can be enhanced by raising the pressure of the high-pressure liquid to jet the liquid at an increased jetting velocity. However, the liquid pressurizing unit 68 must be of a large capacity to pressurize the liquid at a higher pressure, which is not economically advantageous. Concretely, the jetting velocity of the liquid is about 130 m/s when the pressure of the liquid is 100 kgf/cm.sup.2.
FIG. 18 is a sectional view of a conventional two-fluid cleaning jet nozzle 70. The two-fluid cleaning jet nozzle 70 has a first tube 72 for conducting a gas and a second tube 73 for conducting a liquid. The front end portion of the second tube 73 is disposed in the first tube with its axis in parallel to that of the first tube 72.
FIG. 19 is a view of a two-fluid cleaning apparatus for cleaning semiconductor wafers, employing the conventional two-fluid cleaning jet nozzle 70, and having a process cup 8, a stage 6 disposed in the process cup 8 to hold a semiconductor wafer 5, a motor 7 for rotating the stage 6, the two-fluid cleaning jet nozzle 70 for jetting liquid toward the surface of the semiconductor wafer 5, a gas supply means 2a for supplying a pressurized gas to the two-fluid cleaning jet nozzle 70 and a liquid supply means 3a for supplying a pressurized liquid to the two-fluid cleaning jet nozzle 70. A discharge duct 9 is connected to the process cup 8. The two-fluid cleaning jet nozzle 70 is held and moved by a robot arm 4.
In operation, the semiconductor wafer 5 is held fixedly on the stage 6 which is rotated at a predetermined rotating speed. The gas supply means 2a and the liquid supply means 3a supply a pressurized gas and a pressurized liquid, respectively, to the two-fluid cleaning jet nozzle 70. The two-fluid cleaning jet nozzle 70 mixes the gas and the liquid so that the liquid is changed into liquid droplets 1 as shown in FIG. 18. The liquid droplets 1 are accelerated in a section a-b of the first tube 72 by the flow of the gas and are jetted through the front end of the first tube 72 against the surface of the semiconductor wafer 5, as shown in FIG. 19, to remove contaminants adhering thereto. The contaminants removed from the semiconductor wafer 5, the liquid droplets 1 scattered by the surface of the semiconductor wafer 5 and the gas jetted by the two-fluid cleaning jet nozzle 70 are discharged from the process cup 8 through the discharge duct 9. During the cleaning operation, the robot arm 4 holding the two-fluid cleaning jet nozzle 70 moves the two-fluid cleaning jet nozzle 70 horizontally along the surface of the semiconductor wafer 5 to clean the entire surface of the semiconductor wafer 5.
The cleaning ability of the two-fluid cleaning apparatus employing the two-fluid cleaning jet nozzle 70 is higher than that of the foregoing high-pressure jet cleaning apparatus, and its running cost is lower than that of a conventional ice scrubber cleaning apparatus. The two-fluid cleaning apparatus does not break minute or micro patterns on the wafer and does not damage metal films having a relatively low hardness because the cleaning force can be controlled to vary in a wide range. However, the cleaning effect of the two-fluid cleaning apparatus is lower than that of the ice scrubber cleaning apparatus, reasons for which will be described hereinafter.
When cleaning a semiconductor wafer by the two-fluid cleaning jet nozzle 70, the cleaning ability of the two-fluid cleaning jet nozzle 70 is dependent on the velocity of the liquid droplets, and the velocity of the liquid droplets is dependent on the flow rate of the gas, the flow rate of the liquid, the length of the section a-b of the first tube 72 and the sectional area of the bore of the first tube 72. For example, the velocity of the liquid droplets is 224 n/s when the flow rate of the gas is 200 l/min, the flow rate of the liquid is 100 ml/min, the length of the section a-b is 100 mm, and the inside diameter of the first tube 72 is 4.35 mm. Since the dimensions of the two-fluid cleaning jet nozzle 70 is fixed, the velocity of the liquid droplets is dependent on the respective flow rates of the gas and the liquid, particularly on the high flow rate of the gas.
As shown in FIG. 18, the first tube 72 is straight, and the sectional area of a portion of the bore of the first tube 72 corresponding to the front end portion of the second tube 73 is smaller than that of the section a-b of the bore of the first tube 72. Therefore, the flow rate of the gas is limited by the sectional area of the narrowest portion of the bore of the first tube 72 around the front end portion of the second tube 73. Usually, the gas is supplied at a maximum pressure of 10 kgf/cm.sup.2. In the semiconductor device manufacturing industry, the maximum pressure of such a gas is about 7 kgf/cm.sup.2. If the outside diameter of the second tube 73 is 3.2 mm, for example, the sectional area of an annular space around the front end portion of the second tube 73 is 6.8 mm.sup.2 and, if the supply pressure of the gas is 7 kgf/cm.sup.2, the flow rate of the gas is about 200 l/min and the velocity of the liquid droplets is 224 m/s.
Theoretically, the velocity of the gas in the two-fluid cleaning jet nozzle 70 can be increased up to about 330 m/s, substantially equal to the sound velocity, by supplying the gas at a high flow rate, if the front end portion of the two-fluid cleaning jet nozzle 70 is straight, similarly to the section a-b shown in FIG. 18. However, the velocity of the liquid droplets cannot be increased to the sound velocity in the two-fluid cleaning jet nozzle 70 if the maximum supply pressure of the gas is 7 kgf/cm.sup.2. Since cleaning ability is dependent on the velocity of the liquid droplets, the cleaning ability of the two-fluid cleaning jet nozzle 70 is low when the supply pressure of the gas is in the ordinary range of supply pressure.
If the supply pressure of the gas is raised beyond the upper limit of the ordinary range of supply pressure, the flow rate of the gas increases and the velocity of the liquid droplets increases. However, as mentioned above, the maximum velocity of the liquid droplets is limited to the sound velocity. If ice particles used for ice scrubber cleaning and liquid droplets such as water droplets used for two-fluid cleaning are caused to impinge on a surface to be cleaned, the cleaning ability of ice scrubber cleaning is higher than that of two-fluid cleaning because of the difference in physical properties between ice and a liquid such as water. Since the velocity of ice particles for ice scrubber cleaning can be increased to the sound velocity at the maximum, the cleaning ability of the two-fluid cleaning using the two-fluid cleaning jet nozzle 70 is unable to exceed that of ice scrubber cleaning.
The conventional cleaning apparatus shown in FIG. 19 has the following problems. The process cup 8 must have a large exhaust capacity, which is economically disadvantageous. Also, the two-fluid cleaning jet nozzle 70 is held at an angle of 60.degree. or less to the surface of the semiconductor wafer 5 to secure a satisfactory discharge of the used liquid and the used gas, with the results that the cleaning ability of the two-fluid cleaning jet nozzle 70 is insufficient and it is difficult to control the cleaning ability of the two-fluid cleaning jet nozzle 70 to prevent damaging micro patterns. Contaminants removed from the surface of the semiconductor wafer 5 must be discharged together with the liquid droplets and the gas from the process cup 8 through the discharge duct 9 to prevent the contaminants from adhering again to the surface of the semiconductor wafer 5. Therefore, the discharge duct 9 is disposed opposite to the two-fluid cleaning jet nozzle 70 so that the liquid droplets and the gas are discharged satisfactorily. In the cleaning apparatus shown in FIG. 19, the liquid droplets and the gas must be discharged at a discharge rate of about 5 m.sup.3 /min or more.
If the two-fluid cleaning jet nozzle 70 is inclined at an angle of 60.degree. or more to the surface of the semiconductor wafer 5, jets of the liquid droplets and the gas are repelled back by the surface of the semiconductor wafer 5 and the liquid and the gas are scattered upward through the upper opening of the process cup 8 to cause the contaminants removed from the surface of the semiconductor wafer 5 to adhere again to the surface of the semiconductor wafer 5. The force exerted by the liquid droplets on the micro patterns (external force), hence the degree of damage to the micro patterns, is dependent on the angle of impingement of the liquid droplets. The nearer the angle of impingement of the liquid droplets to 90.degree., the higher is the cleaning ability, and the less is the damage to micro patterns. Therefore, it is difficult to control damage to micro patterns due to the difference in the angle of impingement of the liquid droplets on the surface of the semiconductor wafer 5 if the liquid droplets are jetted obliquely against the surface of the semiconductor wafer 5. Such problems arise also when removing contaminants adhering to substrates including liquid crystal display plates and photomasks.
As mentioned above, the ability of the conventional cleaning apparatus to clean semiconductor materials is low and they are unable to remove micro dust particles. Also, the cleaning ability of the cleaning apparatus cannot satisfactorily be controlled to reduce damage to semiconductor materials.